U.S. patent application number 11/128843 was filed with the patent office on 2006-11-16 for rate selection for eigensteering in a mimo communication system.
Invention is credited to Santosh Abraham, Arnaud Meylan, Sanjiv Nanda.
Application Number | 20060256761 11/128843 |
Document ID | / |
Family ID | 37419024 |
Filed Date | 2006-11-16 |
United States Patent
Application |
20060256761 |
Kind Code |
A1 |
Meylan; Arnaud ; et
al. |
November 16, 2006 |
Rate selection for eigensteering in a MIMO communication system
Abstract
Techniques for selecting rates for data transmission on
eigenmodes of a MIMO channel are described. An access point
transmits an unsteered MIMO pilot via the downlink. A user terminal
estimates the downlink channel quality based on the downlink
unsteered MIMO pilot and transmits an unsteered MIMO pilot and
feedback information via the uplink. The feedback information is
indicative of the downlink channel quality. The access point
estimates the uplink channel quality and obtains a channel response
matrix based on the uplink unsteered MIMO pilot, decomposes the
channel response matrix to obtain eigenvectors and channel gains
for the eigenmodes of the downlink, and selects rates for the
eigenmodes based on the estimated uplink channel quality, the
channel gains for the eigenmodes, and the feedback information. The
access point processes data based on the selected rates and
transmits steered data and a steered MIMO pilot on the eigenmodes
with the eigenvectors.
Inventors: |
Meylan; Arnaud; (San Diego,
CA) ; Abraham; Santosh; (San Diego, CA) ;
Nanda; Sanjiv; (Ramona, CA) |
Correspondence
Address: |
QUALCOMM INCORPORATED
5775 MOREHOUSE DR.
SAN DIEGO
CA
92121
US
|
Family ID: |
37419024 |
Appl. No.: |
11/128843 |
Filed: |
May 12, 2005 |
Current U.S.
Class: |
370/338 |
Current CPC
Class: |
H04B 7/0417 20130101;
H04L 25/0398 20130101; H04B 7/0643 20130101; H04L 25/0248 20130101;
H04L 1/0002 20130101; H04B 7/0632 20130101; H04L 1/0026 20130101;
H04L 1/0015 20130101; H04L 2025/03414 20130101; H04L 1/1671
20130101; H04L 25/0228 20130101; H04L 2025/03802 20130101; H04L
25/0204 20130101; H04L 1/06 20130101; H04L 1/0025 20130101; H04L
2025/03426 20130101; H04B 7/0421 20130101 |
Class at
Publication: |
370/338 |
International
Class: |
H04Q 7/24 20060101
H04Q007/24 |
Claims
1. An apparatus comprising: a channel processor to receive a first
pilot via a first communication link and to derive a channel
estimate for the first communication link; and a controller to
receive feedback information indicative of channel quality of a
second communication link and to select rates for eigenmodes of the
second communication link based on the feedback information and the
channel estimate.
2. The apparatus of claim 1, wherein the controller sends a request
for pilot and feedback information, and wherein the first pilot and
the feedback information are sent in response to the request.
3. The apparatus of claim 1, further comprising: a pilot processor
to generate a second pilot for transmission via the second
communication link, and wherein the feedback information is derived
based on the second pilot.
4. The apparatus of claim 1, wherein the channel processor
estimates channel quality of the first communication link based on
the first pilot, and wherein the controller estimates
signal-to-noise-and-interference ratios (SNRs) of the eigenmodes
based on the estimated channel quality of the first communication
link and the feedback information and further selects the rates for
the eigenmodes based on the SNRs of the eigenmodes.
5. The apparatus of claim 4, wherein the channel processor obtains
a channel response matrix and an SNR estimate for the first
communication link based on the first pilot and decomposes the
channel response matrix to obtain channel gains for the eigenmodes,
and wherein the controller estimates the SNRs of the eigenmodes
based on the channel gains for the eigenmodes, the SNR estimate for
the first communication link, and the feedback information.
6. The apparatus of claim 4, wherein the controller selects a rate
for each eigenmode based on an SNR of the eigenmode.
7. The apparatus of claim 4, wherein the controller selects a rate
combination for the eigenmodes based on the SNRs of the
eigenmodes.
8. The apparatus of claim 1, wherein the feedback information
comprises a signal-to-noise-and-interference ratio (SNR) estimate
for the second communication link.
9. The apparatus of claim 1, wherein the feedback information
comprises at least one rate or an overall throughput for the second
communication link.
10. The apparatus of claim 1, wherein the feedback information
comprises acknowledgments or negative acknowledgments for data
packets.
11. The apparatus of claim 1, wherein the first pilot and the
feedback information are received from a single transmission sent
via the first communication link.
12. The apparatus of claim 1, wherein the feedback information is
received for a prior data transmission sent via the second
communication link.
13. The apparatus of claim 1, further comprising: a data processor
to process data based on the rates selected for the eigenmodes; and
a spatial processor to spatially process the data for transmission
on the eigenmodes.
14. The apparatus of claim 1, wherein the first pilot is an
unsteered multiple-input multiple-output (MIMO) pilot sent from a
first plurality of antennas and received via a second plurality of
antennas.
15. A method of performing rate selection, comprising: receiving a
first pilot via a first communication link; receiving feedback
information indicative of channel quality of a second communication
link; and selecting rates for eigenmodes of the second
communication link based on the feedback information and the first
pilot.
16. The method of claim 15, further comprising: sending a request
for pilot and feedback information, and wherein the first pilot and
the feedback information are sent in response to the request.
17. The method of claim 15, further comprising: transmitting a
second pilot via the second communication link, and wherein the
feedback information is derived based on the second pilot.
18. The method of claim 15, wherein the selecting the rates for the
eigenmodes comprises estimating channel quality of the first
communication link based on the first pilot, estimating
signal-to-noise-and-interference ratios (SNRs) of the eigenmodes
based on the estimated channel quality of the first communication
link and the feedback information, and selecting the rates for the
eigenmodes based on the SNRs of the eigenmodes.
19. The method of claim 18, wherein the estimating the SNRs of the
eigenmodes comprises obtaining a channel response matrix for the
first communication link based on the first pilot, decomposing the
channel response matrix to obtain channel gains for the eigenmodes,
and deriving the SNRs of the eigenmodes based on the channel gains
for the eigenmodes, the estimated channel quality of the first
communication link, and the feedback information.
20. An apparatus comprising: means for receiving a first pilot via
a first communication link; means for receiving feedback
information indicative of channel quality of a second communication
link; and means for selecting rates for eigenmodes of the second
communication link based on the feedback information and the first
pilot.
21. The apparatus of claim 20, further comprising: means for
sending a request for pilot and feedback information, and wherein
the first pilot and the feedback information are sent in response
to the request.
22. The apparatus of claim 20, further comprising: means for
transmitting a second pilot via the second communication link, and
wherein the feedback information is derived based on the second
pilot.
23. The apparatus of claim 20, wherein the means for selecting the
rates for the eigenmodes comprises means for estimating channel
quality of the first communication link based on the first pilot,
means for estimating signal-to-noise-and-interference ratios (SNRs)
of the eigenmodes based on the estimated channel quality of the
first communication link and the feedback information, and means
for selecting the rates for the eigenmodes based on the SNRs of the
eigenmodes.
24. The apparatus of claim 23, wherein the means for estimating the
SNRs of the eigenmodes comprises means for obtaining a channel
response matrix for the first communication link based on the first
pilot, means for decomposing the channel response matrix to obtain
channel gains for the eigenmodes, and means for deriving the SNRs
of the eigenmodes based on the channel gains for the eigenmodes,
the estimated channel quality of the first communication link, and
the feedback information.
25. A method of performing rate selection in a multiple-input
multiple-output (MIMO) communication system, comprising:
transmitting a first unsteered MIMO pilot via a downlink; receiving
a second unsteered MIMO pilot and feedback information via an
uplink, wherein the feedback information is indicative of downlink
channel quality estimated based on the first unsteered MIMO pilot;
and selecting rates for eigenmodes of the downlink based on the
feedback information and the second unsteered MIMO pilot.
26. The method of claim 25, wherein the selecting the rates for the
eigenmodes of the downlink comprises estimating uplink channel
quality based on the second unsteered MIMO pilot, obtaining a
channel response matrix for the uplink based on the second
unsteered MIMO pilot, decomposing the channel response matrix to
obtain channel gains for the eigenmodes, estimating
signal-to-noise-and-interference ratios (SNRs) of the eigenmodes
based on the estimated uplink channel quality, the channel gains
for the eigenmodes, and the feedback information, and selecting the
rates for the eigenmodes based on the SNRs of the eigenmodes.
27. An apparatus comprising: a pilot processor to generate a first
pilot for transmission via a first communication link; a controller
to send feedback information indicative of channel quality of a
second communication link; and a spatial processor to receive a
data transmission on eigenmodes of the second communication link,
wherein the data transmission is sent at rates selected based on
the first pilot and the feedback information.
28. The apparatus of claim 27, wherein the controller receives a
request for pilot and feedback information and sends the first
pilot and the feedback information in response to the request.
29. The apparatus of claim 27, further comprising: a channel
processor to receive a second pilot via the second communication
link and to derive a signal-to-noise-and-interference ratio (SNR)
estimate for the second communication link based on the second
pilot, and wherein the controller generates the feedback
information based on the SNR estimate.
30. The apparatus of claim 27, wherein the pilot processor
generates the first pilot as an unsteered multiple-input
multiple-output (MIMO) pilot suitable for transmission from a
plurality of antennas.
31. A method of performing rate selection, comprising: transmitting
a first pilot via a first communication link; sending feedback
information indicative of channel quality of a second communication
link; and receiving a data transmission on eigenmodes of the second
communication link, wherein the data transmission is sent at rates
selected based on the first pilot and the feedback information.
32. The method of claim 31, further comprising: receiving a request
for pilot and feedback information, and wherein the first pilot and
the feedback information are sent in response to the request.
33. The method of claim 31, further comprising: receiving a second
pilot via the second communication link; deriving a
signal-to-noise-and-interference ratio (SNR) estimate for the
second communication link based on the second pilot; and generating
the feedback information based on the SNR estimate.
34. An apparatus comprising: means for transmitting a first pilot
via a first communication link; means for sending feedback
information indicative of channel quality of a second communication
link; and means for receiving a data transmission on eigenmodes of
the second communication link, wherein the data transmission is
sent at rates selected based on the first pilot and the feedback
information.
35. The apparatus of claim 34, further comprising: means for
receiving a request for pilot and feedback information, and wherein
the first pilot and the feedback information are sent in response
to the request.
36. The apparatus of claim 34, further comprising: means for
receiving a second pilot via the second communication link; means
for deriving a signal-to-noise-and-interference ratio (SNR)
estimate for the second communication link based on the second
pilot; and means for generating the feedback information based on
the SNR estimate.
Description
BACKGROUND
[0001] I. Field
[0002] The present invention relates generally to communication,
and more specifically to techniques for selecting rates for data
transmission in a multiple-input multiple-output (MIMO)
communication system.
[0003] II. Background
[0004] A MIMO system employs multiple (T) transmit antennas at a
transmitting station and multiple (R) receive antennas at a
receiving station for data transmission. A MIMO channel formed by
the T transmit antennas and the R receive antennas may be
decomposed into S spatial channels, where S.ltoreq.min {T, R }. The
S spatial channels may be used to transmit data in parallel to
achieve higher throughput and/or redundantly to achieve greater
reliability.
[0005] Each spatial channel may experience various deleterious
channel conditions such as, e.g., fading, multipath, and
interference effects. The S spatial channels may experience
different channel conditions and may achieve different
signal-to-noise-and-interference ratios (SNRs). The SNR of each
spatial channel determines its transmission capacity, which is
typically quantified by a particular data rate that may be reliably
transmitted on the spatial channel.
[0006] Rate selection refers to the process of selecting suitable
rates for data transmission, e.g., on the spatial channels of the
MIMO channel. A "rate" may be associated with a particular data
rate or information bit rate, a particular coding scheme or code
rate, a particular modulation scheme, and so on to use for a data
stream. For a time variant MIMO channel, the channel conditions
change over time and the SNR of each spatial channel also changes
over time. The different SNRs for different spatial channels plus
the time varying nature of the SNR for each spatial channel make it
challenging to select the proper rates for the spatial
channels.
[0007] There is therefore a need in the art for techniques to
select rates in a MIMO system.
SUMMARY
[0008] Techniques for selecting rates for data transmission on
eigenmodes of a MIMO channel are described herein. An eigenmode may
be viewed as an orthogonal spatial channel obtained by decomposing
a channel response matrix for the MIMO channel. The techniques may
be used for downlink data transmission from an access point (AP) to
a user terminal (UT), uplink data transmission from the user
terminal to the access point, and peer-to-peer data transmission
between two user terminals.
[0009] According to an embodiment of the invention, an apparatus is
described which includes a channel processor and a controller. The
channel processor receives a pilot (e.g., an unsteered MIMO pilot)
via a first communication link (e.g., the uplink) and derives a
channel estimate for the first communication link. The controller
receives feedback information indicative of the channel quality of
a second communication link (e.g., the downlink) and selects rates
for eigenmodes of the second communication link based on the
feedback information and the channel estimate.
[0010] According to another embodiment, a method is provided in
which a pilot is received via a first communication link. Feedback
information indicative of the channel quality of a second
communication link is also received. Rates for eigenmodes of the
second communication link are selected based on the feedback
information and the pilot.
[0011] According to yet another embodiment, an apparatus is
described which includes means for receiving a pilot via a first
communication link, means for receiving feedback information
indicative of the channel quality of a second communication link,
and means for selecting rates for eigenmodes of the second
communication link based on the feedback information and the
pilot.
[0012] According to yet another embodiment, a method is provided in
which a first unsteered MIMO pilot is transmitted via the downlink.
A second unsteered MIMO pilot and feedback information are received
via the uplink. The feedback information is indicative of the
downlink channel quality, which is estimated based on the first
unsteered MIMO pilot. Rates for eigenmodes of the downlink are
selected based on the feedback information and the second unsteered
MIMO pilot.
[0013] According to yet another embodiment, an apparatus is
described which includes a pilot processor, a controller, and a
spatial processor. The pilot processor generates a pilot for
transmission via a first communication link. The controller sends
feedback information indicative of the channel quality of a second
communication link. The spatial processor receives a data
transmission on eigenmodes of the second communication link. The
data transmission is sent at rates selected based on the pilot and
the feedback information.
[0014] According to yet another embodiment, a method is provided in
which a pilot is transmitted via a first communication link.
Feedback information indicative of the channel quality of a second
communication link is also sent. A data transmission, which is sent
at rates selected based on the pilot and the feedback information,
is received on eigenmodes of the second communication link.
[0015] According to yet another embodiment, an apparatus is
described which includes means for transmitting a pilot via a first
communication link, means for sending feedback information
indicative of the channel quality of a second communication link,
and means for receiving a data transmission on eigenmodes of the
second communication link. The data transmission is sent at rates
selected based on the pilot and the feedback information.
[0016] Various aspects and embodiments of the invention are
described in detail below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 shows a scheme for AP-initiated data transmission on
eigenmodes.
[0018] FIG. 2 shows a process for transmitting data on eigenmodes
with low overhead.
[0019] FIGS. 3 and 4 show two schemes for AP-initiated data
transmission on eigenmodes with low overhead.
[0020] FIG. 5 shows a scheme for UT-initiated data transmission on
eigenmodes.
[0021] FIG. 6 shows a scheme for UT-initiated data transmission on
eigenmodes with low overhead.
[0022] FIG. 7 shows a process for transmitting data on eigenmodes
with low overhead.
[0023] FIG. 8 shows a block diagram of an access point and a user
terminal.
DETAILED DESCRIPTION
[0024] The word "exemplary" is used herein to mean "serving as an
example, instance, or illustration." Any embodiment described
herein as "exemplary" is not necessarily to be construed as
preferred or advantageous over other embodiments.
[0025] The rate selection techniques described herein may be used
for data transmission on the downlink and uplink. The downlink (or
forward link) refers to the communication link from an access point
to a user terminal, and the uplink (or reverse link) refers to the
communication link from the user terminal to the access point. For
clarity, much of the following description is for downlink data
transmission from the access point to the user terminal. An access
point may also be called a base station, a base transceiver
station, and so on. A user terminal may also be called a mobile
station, a user equipment, a wireless device, and so on.
[0026] A downlink MIMO channel formed by T antennas at the access
point and R antennas at the user terminal may be characterized by
an R.times.T channel response matrix H, which may be expressed as:
H _ = [ h 1 , 1 h 1 , 2 h 1 , T h 2 , 1 h 2 , 2 h 2 , T h R , 1 h R
, 2 h R , T ] , Eq .times. .times. ( 1 ) ##EQU1## where entry
h.sub.i,j, for i=1, . . . , R and j=1, . . . , T, is the coupling
or complex gain between AP antenna j and UT antenna i. For
simplicity, the MIMO channel is assumed to be flat fading, and the
coupling between each pair of AP and UT antennas is represented
with a single complex gain h.sub.i,j.
[0027] The channel response matrix H may be diagonalized to obtain
S eigenmodes or orthogonal spatial channels of the downlink MIMO
channel. This diagonalization may be achieved by performing either
singular value decomposition of H or eigenvalue decomposition of a
correlation matrix of H, which is R=H.sup.HH, where H.sup.H denotes
the conjugate transpose of H. For clarity, singular value
decomposition is used in the following description. The singular
value decomposition of H may be expressed as: H=UXV.sup.H, Eq (2)
where U is an R x R unitary matrix of left eigenvectors of H;
[0028] .SIGMA. is an R.times.T diagonal matrix of singular values
of H; and [0029] V is a T.times.T unitary matrix of right
eigenvectors of H.
[0030] A unitary matrix Q is characterized by the property
Q.sup.HQ=I, where I is the identity matrix. The columns of a
unitary matrix are orthogonal to one another, and each column has
unit power. The right eigenvectors in V may be used for spatial
processing to transmit data on the eigenmodes of H. The left
eigenvectors in U may be used for receiver spatial processing to
recover the data transmission sent on the eigenmodes of H. The
diagonal matrix .SIGMA. contains non-negative real values along the
diagonal and zeros elsewhere. These diagonal entries are referred
to as singular values of H and represent the channel gains for the
eigenmodes. Singular value decomposition is described by Gilbert
Strang in "Linear Algebra and Its Applications," Second Edition,
Academic Press, 1980.
[0031] The access point performs spatial processing for
eigensteering as follows: x=Vs , Eq(3) where s is a vector with up
to S data symbols to be sent on the S eigenmodes; and [0032] x is a
vector with T transmit symbols to be sent from the T AP antennas.
Eigensteering refers to transmission of data on eigenmodes of a
MIMO channel.
[0033] As used herein, a "data symbol" is a modulation symbol for
data, a "pilot symbol" is a modulation symbol for pilot, a
"transmit symbol" is a symbol to be sent from a transmit antenna, a
"received symbol" is a symbol obtained from a receive antenna, and
a symbol is a complex value. A pilot is a transmission that is
known a priori by both the transmitting and receiving stations. A
pilot may also be referred to as sounding, training, a reference
transmission, a preamble, and so on. For clarity, the following
description assumes that one data stream is sent on each
eigenmode.
[0034] The received symbols at the user terminal may be expressed
as: r=Hx+n=HVs+n=H.sub.effs+n, Eq(4) where r is a vector with R
received symbols from the R UT antennas; [0035] H.sub.eff=HV is an
effective MIMO channel response matrix for vector s; and [0036] n
is a noise vector. For simplicity, the noise is assumed to be
additive white Gaussian noise (AWGN) with a zero mean vector and a
covariance matrix of .phi..sub.nn=.sigma..sub.noise.sup.2I, where
.sigma..sub.noise .sup.2 is the variance of the noise. The user
terminal may recover the transmitted data symbols using various
receiver spatial processing techniques such as a full-CSI
technique, a minimum mean square error (MMSE) technique, and a
zero-forcing (ZF) technique.
[0037] The user terminal may derive a spatial filter matrix based
on the full-CSI, MMSE, or zero-forcing technique, as follows:
M.sub.fcsi=.SIGMA..sup.-1U.sup.H=H.sub.eff.sup.H, Eq (5)
M.sub.mmse=D.sub.mmse[H.sub.eff.sup.HH.sub.eff+.sigma..sub.noise.sup.2I].-
sup.-1H.sub.eff.sup.H, Eq (6)
M.sub.zf=[H.sub.eff.sup.HH.sub.eff].sup.-1H.sub.eff.sup.H=R.sub.eff.sup.--
1H.sub.eff.sup.H, Eq (7)
whereD.sub.mmse=diag{[H.sub.eff.sup.HH.sub.eff+.sigma..sub.noise.sup.2I].-
sup.-1H.sub.eff.sup.HH.sub.eff}.sup.-1,
[0038] The user terminal may perform receiver spatial processing as
follows: s=Mr=s+n, Eq (8) where M is a spatial filter matrix, which
may be equal to M.sub.fcsi, M.sub.mmse or M.sub.zf; [0039] s is a
vector with up to S detected data symbols; and [0040] n is the
noise after the receiver spatial processing. The detected data
symbols in s are estimates of the transmitted data symbols in
s.
[0041] The SNR of each eigenmode m, for m=1, . . . , S, may be
expressed as: SNR fcsi , m = 10 .times. log 10 .function. ( P m
.sigma. m 2 .sigma. noise 2 ) , Eq .times. .times. ( 9 ) SNR mmse ,
m = 10 .times. log 10 .function. ( q m 1 - q m P m ) , Eq .times.
.times. ( 10 ) SNR zf , m = 10 .times. log 10 .function. ( P m r m
.sigma. noise 2 ) , Eq .times. .times. ( 11 ) ##EQU2## where
P.sub.m is the transmit power used for eigenmode m; [0042]
.sigma..sub.m is the singular value for eigenmode m, which is the
m-th diagonal element of .SIGMA.; [0043] q.sub.m is the m-th
diagonal element of D.sub.mmse.sup.-1; and [0044] r.sub.m is the
m-th diagonal element of R.sub.eff.sup.-1. SNR.sub.fcsi,m,
SNR.sub.mmse,m, and SNR.sub.zf,m are the SNR of eigenmode m for the
full-CSI, MMSE, and zero-forcing techniques, respectively, and are
in units of decibel (dB). The term P.sub.m/.sigma..sub.noise.sup.2
is often referred to as the received SNR. The terms SNR.sub.fcsi,m,
SNR.sub.mmse,m, and SNR.sub.zf,m are often referred to as the
post-detection SNRs, which are the SNRs after the receiver spatial
processing.
[0045] The rates for the eigenmodes may be selected based on the
SNRs of these eigenmodes. The rate selection is dependent on the
rate selection scheme supported by the system. In one rate
selection scheme, the system allows a rate to be independently
selected for each eigenmode based on the SNR of that eigenmode. The
system may support a set of rates, and each supported rate may be
associated with a particular minimum SNR required to achieve a
specified level of performance, e.g., 1% packet error rate (PER).
The required SNR for each supported rate may be obtained by
computer simulation, empirical measurements, and so on. The set of
supported rates and their required SNRs may be stored in a look-up
table. The SNR of each eigenmode, SNR.sub.m, may be compared
against the required SNRs for the supported rates to determine the
highest rate R.sub.m supported by that SNR.sub.m. The rate R.sub.m
selected for each eigenmode is associated with the highest data
rate and a required SNR that is less than or equal to SNR.sub.m, or
SNR.sub.req (R.sub.m).ltoreq.SNR.sub.m.
[0046] In another rate selection scheme, the system allows only
certain combinations of rates to be used for data transmission. The
set of rate combinations allowed by the system is often called a
vector-quantized rate set. A rate combination may also be called a
modulation coding scheme (MCS) or some other terminology. Each
allowed rate combination is associated with a specific number of
data streams to transmit, a specific rate for each data stream, and
an overall throughput for all of the data streams. The SNRs of the
eigenmodes may be used to select one of the allowed rate
combinations.
[0047] The access point uses the following information to transmit
data on the eigenmodes of the downlink MIMO channel: [0048] a set
of right eigenvectors in V; and [0049] a set of rates for data
streams sent on the eigenmodes. Different rates may be used for
different eigenmodes since these eigenmodes may achieve different
SNRs. The access point may obtain the eigenvectors and the rates
for the eigenmodes in various manners.
[0050] In a time division duplex (TDD) system, the downlink and
uplink share the same frequency band, and the downlink and uplink
channel responses may be assumed to be reciprocal of one another.
That is, if H is the channel response matrix from antenna array X
to antenna array Y, then a reciprocal channel implies that the
coupling from array Y to array X is given by H.sup.T, where H.sup.T
denotes the transpose of H. However, the responses of the transmit
and receive chains at the access point are typically different from
the responses of the transmit and receive chains at the user
terminal. Calibration may be performed to derive correction
matrices that can account for differences in the responses of the
transmit/receive chains at the two stations. The application of the
correction matrices at these two stations allows a calibrated
channel response for one link to be expressed as a transpose of a
calibrated channel response for the other link. For simplicity, the
following description assumes a flat frequency response for the
transmit/receive chains. The downlink channel response matrix is
H.sub.dl=H, and the uplink channel response matrix is
H.sub.ul=H.sup.T.
[0051] The singular value decomposition of H.sub.dl and H.sub.ul
may be expressed as: H.sub.dl=U.SIGMA.V.sup.H and
H.sub.ul=V*.SIGMA..sup.TU.sup.T, Eq (12) where V* is a complex
conjugate of V. As shown in equation (12), U and V are matrices of
left and right eigenvectors of H.sub.dl, and V* and U* are matrices
of left and right eigenvectors of H.sub.ul.
[0052] The access point performs spatial processing with V to
transmit data on eigenmodes to the user terminal. The user terminal
performs receiver spatial processing with U.sup.H (or H and V) to
recover the downlink data transmission. One station may transmit an
unsteered MIMO pilot that may be used by the other station to
obtain an estimate of H. An unsteered MIMO pilot is a pilot
comprised of N pilot transmissions sent from N antennas, where the
pilot transmission from each antenna is identifiable by the
receiving station. N=T for a downlink unsteered MIMO pilot sent by
the access point, and N=R for an uplink unsteered MIMO pilot sent
by the user terminal. The transmitting station may orthogonalize
the N pilot transmissions in (1) code domain by using a different
orthogonal sequence (e.g., Walsh sequence) for each pilot
transmission, (2) frequency domain by sending each pilot
transmission on a different frequency subband, or (3) time domain
by sending each pilot transmission in a different time interval. In
any case, the receiving station is able to obtain an estimate of H
based on the unsteered MIMO pilot received from the transmitting
station. For simplicity, the following description assumes no
errors in channel estimation.
[0053] Singular value decomposition is computationally intensive.
Thus, it may be desirable to have the access point perform singular
value decomposition of H to obtain the eigenvectors in V. The
access point may then transmit a steered MIMO pilot, which is a
pilot sent on the eigenmodes of the MIMO channel. The steered MIMO
pilot may be generated as follows: x.sub.pilot,m=v.sub.mp.sub.m, Eq
(13) where v.sub.m is the right eigenvector for eigenmode m, which
is the m-th column of V; [0054] p.sub.m is a pilot symbol
transmitted on eigenmode m; and [0055] x.sub.pilot,m is a transmit
vector for the steered MIMO pilot for eigenmode m. The access point
may transmit a complete steered MIMO pilot on all eigenmodes in one
or multiple (consecutive or non-consecutive) symbol periods.
[0056] The received steered MIMO pilot at the user terminal may be
expressed as:
R.sub.pilot,m=Hx.sub.pilot,m+n=U.SIGMA.V.sup.Hv.sub.mp.sub.m+n=u.sub.m.si-
gma..sub.mp.sub.m+n, Eq (14) where r.sub.pilot,m is a received
vector for the steered MIMO pilot for eigenmode m; and [0057]
u.sub.m is the left eigenvector for eigenmode m, which is the m-th
column of U. Equation (14) indicates that the user terminal may
obtain (1) an estimate of U, one column at a time, and (2) an
estimate of .SIGMA., one singular value .sigma..sub.m at a time,
based on the steered MIMO pilot from the access point. The user
terminal can obtain estimates of the eigenvectors and the singular
values without having to perform singular value decomposition.
[0058] The user terminal typically selects the rates for the
eigenmodes of the downlink MIMO channel and sends the selected
rates back to the access point. The access point typically cannot
select the rates for the downlink MIMO channel based solely on an
uplink MIMO pilot from the user terminal because of various factors
such as, e.g., (1) different receiver noise levels at the access
point and the user terminal, (2) different interference levels
observed by the access point and the user terminal, and/or (3)
different transmit powers used for uplink MIMO pilot and downlink
data transmission.
[0059] FIG. 1 shows an exemplary pilot and data transmission scheme
100 to transmit data on eigenmodes of the downlink MIMO channel.
The timeline for the access point and the timeline for the user
terminal are not necessarily drawn to scale in FIG. 1.
[0060] Initially, the access point sends to the user terminal a
request for a pilot, which may be called a pilot request (Pilot
Req) or a training request (TRQ) (block 110). The user terminal
receives the pilot request and, in response, transmits an unsteered
MIMO pilot in a sounding packet (block 112). The access point
receives the unsteered MIMO pilot, estimates the channel response
matrix H, and decomposes H to obtain eigenvectors. The access point
then transmits a steered MIMO pilot and a request for rate
feedback, which may be called a rate request (Rate Req) or an MCS
request (MRQ) (block 114). The user terminal receives the steered
MIMO pilot, estimates the SNR of each eigenmode based on the
steered MIMO pilot, and selects rates for the eigenmodes based on
the SNRs of the eigenmodes. The user terminal then sends back the
selected rates for the eigenmodes (block 116). The access point
receives the selected rates from the user terminal, processes
(e.g., encodes and modulates) data based on the selected rates, and
spatially processes the data based on the eigenvectors. The access
point then transmits a steered MIMO pilot and steered data to the
user terminal (block 118).
[0061] Transmission scheme 100 allows the access point to transmit
data at the proper rates on the eigenmodes of H without the need
for the user terminal to perform singular value decomposition.
However, four overhead transmissions are needed for blocks 110
through 116 in order to transmit data with eigensteering in block
118. The four overhead transmissions may wipe out the gain in
higher overall throughput achieved with eigensteering. As an
example, for a system in which the four overhead transmissions
require 264 microseconds (.mu.s), when the raw data rate with
eigensteering is assumed to be 140 Mbps, and the raw data rate
without eigensteering is assumed to be 33% lower, the payload size
should exceed 8 kilobytes in order to amortize the overhead for
eigensteering. For smaller payload sizes, performance is better
without eigensteering because of lower overhead.
[0062] FIG. 2 shows a process 200 for transmitting data on
eigenmodes of the downlink MIMO channel with low overhead.
Initially, the access point sends a request for pilot and feedback
information, e.g., TRQ and MRQ (block 212). The access point also
transmits a downlink (DL) unsteered MIMO pilot, e.g., along with
the request for pilot and feedback information (block 214).
[0063] The user terminal receives and processes the downlink
unsteered MIMO pilot and estimates the downlink channel quality,
which may be quantified as described below (block 216). The user
terminal then sends feedback information indicative of the downlink
channel quality to the access point (block 218). The user terminal
also transmits an uplink (UL) unsteered MIMO pilot, e.g., along
with the feedback information (block 220).
[0064] The access point receives the uplink unsteered MIMO pilot,
estimates the channel response matrix H based on the unsteered MIMO
pilot, and decomposes H to obtain eigenvectors V and singular
values for the eigenmodes of H (block 222). The access point also
processes the uplink unsteered MIMO pilot and estimates the uplink
channel quality (block 224). The access point then estimates the
SNRs of the eigenmodes based on the estimated uplink channel
quality, the singular values, and the feedback information from the
user terminal, as described below (block 226). The access point
selects the rates for the eigenmodes based on the estimated SNRs of
the eigenmodes (block 228). The access point then processes (e.g.,
encodes and modulates) data based on the selected rates to obtain
data symbols (block 230). The access point performs spatial
processing on the data symbols with the eigenvectors V, e.g., as
shown in equation (3), and transmits steered data and a downlink
steered MIMO pilot on the eigenmodes to the user terminal (block
232). The access point informs the user terminal of the rates used
for the current downlink data transmission.
[0065] The user terminal receives the downlink steered MIMO pilot
and estimates the effective channel response matrix H.sub.eff
(block 234). The user terminal then performs receiver spatial
processing on the downlink data transmission with H.sub.eff, e.g.,
as shown in equations (5) through (8) (block 236). The user
terminal processes (e.g., demodulates and decodes) the detected
data symbols based on the rates selected by the access point to
obtain decoded data (block 238).
[0066] FIG. 3 shows an improved pilot and data transmission scheme
300 that may be used for process 200 in FIG. 2. For scheme 300, the
access point sends a request for pilot and feedback information and
a downlink unsteered MIMO pilot in a first overhead transmission
(block 310). The first overhead transmission may be, e.g., an
Initiator Aggregate Control (IAC) message with the training request
(TRQ) and MCS request (MRQ) fields being set (or IAC+TRQ+MRQ). The
user terminal sends an uplink unsteered MIMO pilot and feedback
information in a second overhead transmission (block 312). The
second overhead transmission may be, e.g., a Responder Aggregate
Control (RAC) message with an MCS feedback field (MFB) being set
and further including a sounding packet (or RAC+MFB+sounding
packet). The access point then transmits a steered MIMO pilot and
steered data to the user terminal (block 314).
[0067] Transmission scheme 300 allows the access point to transmit
data at the proper rates on the eigenmodes of H using only two
overhead transmissions. Comparing scheme 300 in FIG. 3 to scheme
100 in FIG. 1, blocks 110 and 114 in FIG. 1 are essentially
combined into block 310 in FIG. 3, and blocks 112 and 116 in FIG. 1
are essentially combined into block 312 in FIG. 3. The major
difference between the two schemes is that the user terminal sends
back (1) the rates for the eigenmodes in block 116 (since a steered
MIMO pilot is available) and (2) feedback information for the
downlink MIMO channel in block 312 (since an unsteered MIMO pilot
is available). For scheme 300, the access point performs additional
processing to select the rates for data transmission on the
eigenmodes of the downlink.
[0068] In block 214 in FIG. 2 and block 310 in FIG. 3, the access
point sends T pilot transmissions from T AP antennas for the
downlink unsteered MIMO pilot. The user terminal may estimate the
SNR for each AP antenna based on the pilot transmission received
from that AP antenna. The SNRs for the T AP antennas are called
downlink SNRs and are denoted as SNR.sub.dl,i for i=1, . . . ,
T.
[0069] The user terminal may send the feedback information in
various forms. In an embodiment, the feedback information comprises
quantized values of the downlink SNRs. In another embodiment, the
user terminal derives an average downlink SNR as follows: SNR dl =
1 T i = 1 T .times. .times. SNR dl , i . Eq .times. .times. ( 15 )
##EQU3## The feedback information then comprises a quantized value
of SNR.sub.dl. The downlink SNRs and the average downlink SNR are
different forms of an SNR estimate for the downlink.
[0070] In yet another embodiment, the user terminal selects a set
of rates based on the downlink SNRs. The feedback information
comprises the selected rates, which may be viewed as coarse
quantized values of the downlink SNRs. In yet another embodiment,
the user terminal selects a single rate based on the average
downlink SNR, and the feedback information comprises the selected
rate. In yet another embodiment, the user terminal selects a rate
combination based on the downlink SNRs, and the feedback
information comprises the selected rate combination. In yet another
embodiment, the feedback information comprises an overall
throughput for the selected rates or the selected rate combination.
In yet another embodiment, the feedback information comprises a
noise floor or noise variance .sigma..sub.noise.sup.2 observed at
the user terminal.
[0071] In yet another embodiment, the feedback information
comprises acknowledgments (ACKs) and/or negative acknowledgments
(NAKs) sent by the user terminal for data packets received from the
access point. The access point may maintain a power control loop
that adjusts a target SNR for the user terminal based on the
received ACKs/NAKs. The access point may use the target SNR to
select the appropriate rates for downlink transmission, as
described below.
[0072] In general, the feedback information may comprise any type
of information that is indicative of the downlink channel quality.
The feedback information may comprise information sent by one or
more layers such as a physical layer, a link layer, and so on.
[0073] The feedback information may be sent in various manners. In
an embodiment, the feedback information is sent in a message having
the proper format and fields. This message may be a control message
at the link layer and may be sent whenever there is feedback
information to send. In another embodiment, the feedback
information is sent in one or more designated fields of a frame or
packet, e.g., at the physical layer. The designated fields may be
available in each frame or packet that is transmitted and may be
set whenever there is feedback information to send.
[0074] The access point may select the rates for the eigenmodes in
various manners, e.g., depending on the type of feedback
information received from the user terminal. To simplify the rate
selection by the access point, the noise and interference at the
user terminal may be assumed to be approximately constant across
spatial dimension, and the noise and interference at the access
point may also be assumed to be approximately constant across
spatial dimension.
[0075] In an embodiment, the SNRs of the eigenmodes are estimated
as follows:
SNR.sub.fcsi,dl,m=SNR.sub.fcsi,ul,m-(SNR.sub.ul-SNR.sub.dl), Eq
(16) where SNR.sub.dl is an SNR estimate for the downlink; [0076]
SNR.sub.ul is an SNR estimate for the uplink; [0077]
SNR.sub.fcsi,ul,m is the SNR of eigenmode m on the uplink; and
[0078] SNR.sub.fcsi,dl,m is an estimate of the SNR of eigenmode m
on the downlink. The SNRs in equation (16) are all in units of dB.
The access point may obtain SNR.sub.dl based on the feedback
information from the user terminal and may obtain SNR.sub.ul based
on the uplink unsteered MIMO pilot. The access point may obtain
SNR.sub.fcsi,ul,m for each eigenmode by (1) decomposing H to obtain
the singular values of H and (2) computing SNR.sub.fcsi,ul,m for
m=1, . . . S, e.g., as shown in equation (9), where
.sigma..sub.noise.sup.2 is the noise variance at the access
point.
[0079] In another embodiment, the SNRs of the eigenmodes are
estimated as follows:
SNR.sub.fcsi,dl,m=SNR.sub.fcsi,ul,m-(SNR.sub.ul-SNR.sub.dl)-SNR-
.sub.bo, Eq (17) where SNR.sub.bo is a back-off factor used to
account for estimation errors. The back-off factor may be selected
based on various considerations such as, e.g., the type of feedback
information (e.g., SNRs or rates) sent by the user terminal, the
age of the feedback information, and so on.
[0080] In another embodiment, the feedback information comprises
one or more rates selected by the user terminal. The access point
may convert the rates to SNRs and then compute an average downlink
SNR based on the converted SNRs, as shown in equation (15). The
access point may then use the average downlink SNR to estimate the
SNRs of the eigenmodes, e.g., as shown in equation (16) or (17). In
yet another embodiment, the feedback information comprises an
overall throughput for the downlink. The access point may convert
the overall throughput to an overall downlink SNR. The access point
may also derive an overall uplink SNR and may use the overall
downlink and uplink SNRs to estimate the SNRs of the
eigenmodes.
[0081] In yet another embodiment, the SNRs of the eigenmodes are
estimated as follows: SNR.sub.fcsi,dl,m=SNR.sub.fcsi,ul,m+ASYM(AP,
UT), Eq (18) where ASYM (AP, UT) is an asymmetric parameter that
indicates the difference in received SNR at the user terminal when
the access point transmits at a known power level on a known
channel to the user terminal. For example, the access point may be
equipped with four antennas, transmit at 17 dBm, and have a noise
figure of 6 dB. The user terminal may be equipped with two
antennas, transmit at 10 dBm, and have a noise figure of 10 dB. The
RSL observed at the user terminal when the access point transmits
at full power on a lossless channel may be computed as:
RSL(AP.fwdarw.UT)=17 dBm-10 dB-10 log.sub.10(2)=10 dBm. Eq (19) The
RSL observed at the access point when the user terminal transmits
at full power on a lossless channel may be computed as:
RSL(UT.fwdarw.AP)=14 dBm-6 dB-10 log.sub.10(4)=14 dBm. Eq (20) The
asymmetric parameter ASYM (AP, UT) may then be computed as:
ASYM(AP, UT)=RSL(AP.fwdarw.UT)-RSL(UT.fwdarw.AP)=-4 dBm. Eq (21)
The asymmetric parameter may also be determined based on the
received SNRs at the access point and the user terminal, as
follows: ASYM(AP, UT)=SNR.sub.ut-SNR.sub.ap, Eq (22) where
SNR.sub.ap is an SNR estimate for the uplink, and [0082] SNR.sub.ut
is an SNR estimate for the downlink. The access point may obtain
SNR.sub.ap based on an unsteered MIMO pilot received from the user
terminal. The access point may derive SNR.sub.ut based on feedback
information (e.g., SNR, rate, ACK/NAK, and so on) sent by the user
terminal. For example, SNR.sub.ut may be a target SNR that is
adjusted based on ACKs/NAKs received from the user terminal.
[0083] In general, the SNRs of the eigenmodes may be estimated in
various manners based on the feedback information and the uplink
unsteered MIMO pilot received from the user terminal. The access
point selects the rates for the eigenmodes based on the SNRs of the
eigenmodes. The access point may select a rate for each eigenmode
based on the SNR of that eigenmode. The access point may also
select a rate combination based on the SNRs of all eigenmodes.
[0084] For scheme 300 in FIG. 3, the user terminal transmits an
unsteered MIMO pilot and feedback information in a single overhead
transmission. The user terminal may also send the pilot and
feedback information separately.
[0085] FIG. 4 shows another pilot and data transmission scheme 400
for eigensteering on the downlink with low overhead. Scheme 400 may
be used for a scenario in which the access point already has
feedback information from the user terminal, e.g., from a prior
data transmission to the user terminal. For scheme 400, the access
point sends a request for pilot (block 410). The user terminal
receives the pilot request and, in response, transmits an uplink
unsteered MIMO pilot (block 412). The access point estimates the
channel response matrix H based on the uplink unsteered MIMO pilot,
decomposes H to obtain the eigenvectors and singular values, and
derives an uplink SNR estimate based on the uplink unsteered MIMO
pilot. The access point selects the rates for the eigenmodes based
on the singular values, the uplink SNR estimate, and the feedback
information already available at the access point. The access point
may use an appropriate back-off factor in equation (17) to account
for the age of the feedback information. For example, a
progressively larger back-off factor may be used for progressively
older feedback information. The access point then transmits a
steered MIMO pilot and steered data to the user terminal (block
414).
[0086] Schemes 100, 300 and 400 are for downlink data transmission
initiated by the access point. Data transmission on the downlink
may also be initiated by the user terminal.
[0087] FIG. 5 shows a pilot and data transmission scheme 500 for
UT-initiated steered data transmission on the downlink. For scheme
500, the user terminal sends a request for downlink data
transmission and an uplink unsteered MIMO pilot (block 512). The
access point derives eigenvectors based on the uplink unsteered
MIMO pilot, sends a request for pilot and rate information, and
transmits a downlink steered MIMO pilot (block 514). The user
terminal estimates the SNRs of the eigenmodes based on the downlink
steered MIMO pilot, selects the rates for the eigenmodes based on
the SNRs of the eigenmodes, and sends the selected rates (block
516). The access point processes data based on the rates selected
by the user terminal and transmits a downlink steered MIMO pilot
and steered data to the user terminal (block 518). Scheme 500 in
FIG. 5 is similar to scheme 100 in FIG. 1, except that the downlink
data transmission is initiated by a data request sent by the user
terminal in scheme 500 instead of a pilot request sent by the
access point in scheme 100. Scheme 500 requires three overhead
transmissions to support eigensteering on the downlink.
[0088] FIG. 6 shows an improved pilot and data transmission scheme
600 for UT-initiated steered data transmission on the downlink with
low overhead. For scheme 600, the user terminal sends a request for
downlink data transmission, an uplink unsteered MIMO pilot, and
possibly feedback information (block 612). The feedback information
may comprise (1) the noise floor or noise variance
.sigma..sub.noise.sup.2 observed by the user terminal, (2) SNRs of
the eigenmodes estimated in a prior downlink data transmission, or
(3) some other information indicative of the downlink channel
quality. The access point derives eigenvectors and singular values
based on the uplink unsteered MIMO pilot and selects the rates for
the eigenmodes based on the uplink unsteered MIMO pilot and the
feedback information. The access point then processes data based on
the selected rates and transmits a downlink steered MIMO pilot and
steered data to the user terminal (block 614). Scheme 600 requires
a single overhead transmission to support eigensteering on the
downlink.
[0089] The rate selection techniques may be used for downlink data
transmission from the access point to the user terminal, as
described above. These techniques may also be used for uplink data
transmission from the user terminal to the access point and for
peer-to-peer data transmission, e.g., from one user terminal to
another user terminal. In general, the transmitting station may be
an access point or a user terminal, and the receiving station may
also be an access point or a user terminal. Using the techniques
described herein, only the transmitting station needs to perform
decomposition, and low overhead is required for eigensteering.
[0090] FIG. 7 shows a process 700 for transmitting data on
eigenmodes with low overhead. A pilot (e.g., an unsteered MIMO
pilot) is received via a first communication link (e.g., the
uplink) (block 712). Feedback information indicative of the channel
quality of a second communication link (e.g., the downlink) is also
received (block 714). The pilot and feedback information may be
received from a single transmission or multiple transmissions. The
pilot and feedback information may be sent in response to a request
for pilot and feedback information, as shown in FIG. 3, or may be
sent along with a data request, as shown in FIG. 6. The feedback
information may be derived based on a pilot transmitted via the
second communication link, as shown in FIG. 3. In any case, the
rates for the eigenmodes of the second communication link are
selected based on the feedback information and the pilot received
via the first communication link, e.g., as described above for
process 200 in FIG. 2 (block 716). Data is processed based on the
selected rates and transmitted on the eigenmodes of the second
communication link (block 718).
[0091] The rate selection techniques described herein may be used
for single-carrier and multi-carrier MIMO systems. Multiple
carriers may be provided by orthogonal frequency division
multiplexing (OFDM) or some other constructs. OFDM effectively
partitions the overall system bandwidth into multiple (K)
orthogonal subbands, which are also called tones, subcarriers,
bins, and frequency channels. With OFDM, each subband is associated
with a respective subcarrier that may be modulated with data.
[0092] For a MIMO system that utilizes OFDM, a channel response
matrix H(k) may be obtained for each subband k and decomposed to
obtain the eigenmodes of that subband. The singular values in each
diagonal matrix .SIGMA.(k), for k=1, . . . , K, may be ordered such
that the first column contains the largest singular value, the
second column contains the next largest singular value, and so on,
or .sigma..sub.1(k).gtoreq..sigma..sub.2(k).gtoreq.. . .
.gtoreq..sigma..sub.s(k), where .sigma..sub.m (k) is the singular
value in the m-th column of .SIGMA.(k) after the ordering. When the
singular values in each matrix .SIGMA.(k) are ordered, the
eigenvectors (or columns) of the matrix V(k) for that subband are
also ordered correspondingly. A wideband eigenmode may be defined
as the set of same-order eigenmode of all K subbands after the
ordering, e.g., wideband eigenmode m includes eigenmode m of all K
subbands. Each wideband eigenmode is associated with a set of K
eigenvectors for the K subbands. The rate selection may be
performed for the S wideband eigenmodes, e.g., similar to that
described above for a single-carrier MIMO system.
[0093] FIG. 8 shows a block diagram of an access point 810 and a
user terminal 850. At access point 810, a data/pilot processor 820
receives traffic data from a data source 812, processes (e.g.,
encodes, interleaves, and modulates) the traffic data, and provides
data symbols. One data stream may be sent on each eigenmode, and
each data stream may be encoded and modulated based on the rate
selected for that stream/eigenmode. Processor 820 also generates
pilot symbols for unsteered and steered MIMO pilots. A transmit
(TX) spatial processor 830 performs spatial processing on the data
and pilot symbols with eigenvectors and provides T streams of
transmit symbols to T transmitter units (TMTR) 832a through 832t.
Each transmitter unit 832 conditions a respective transmit symbol
stream and generates a corresponding modulated signal. T modulated
signals from transmitter units 832a through 832t are transmitted
from T antennas 834a through 834t, respectively.
[0094] At user terminal 850, R antennas 852a through 852r receive
the modulated signals transmitted by access point 810, and each
antenna provides a received signal to a respective receiver unit
(RCVR) 854. Each receiver unit 854 performs processing
complementary to that performed by transmitter units 832 and
provides received symbols. A receive (RX) spatial processor 860
performs spatial matched filtering on the received symbols from all
R receiver units 854 based on a spatial filter matrix and provides
detected data symbols. An RX data processor 870 processes (e.g.,
demodulates, deinterleaves, and decodes) the detected data symbols
and provides decoded data.
[0095] Controllers 840 and 880 control the operation of various
processing units at access point 810 and user terminal 850,
respectively. Memory units 842 and 882 store data and program codes
used by controllers 840 and 880, respectively.
[0096] For rate selection, a channel processor 878 estimates the
downlink channel quality and provides a downlink channel quality
estimate. Controller 880 provides feedback information indicative
of the downlink channel quality. The feedback information and pilot
symbols for an unsteered MIMO pilot are processed by a data/pilot
processor 890 and a TX spatial processor 892 to generate R transmit
symbol streams. R transmitter units 854a through 854r condition the
R transmit symbol streams and generate R modulated signals, which
are sent via R antennas 852a through 852r.
[0097] At access point 810, the modulated signals from user
terminal 850 are received by T antennas 834 and processed by T
receiver units 832 to obtain received symbols. The received symbols
are further processed by an RX spatial processor 844 and an RX data
processor 842 to obtain the feedback information from user terminal
850. A channel processor 838 receives the unsteered MIMO pilot from
user terminal 850 and derives a channel estimate for the uplink.
The channel estimate may comprise a channel response matrix H and
an uplink channel quality estimate. Channel processor 838
decomposes H to obtain eigenvectors and singular values for the
eigenmodes of H and provides the eigenvectors to TX spatial
processor 830. Controller 840 receives the uplink channel quality
estimate and the singular values from channel processor 838 and the
feedback information from RX data processor 846, estimates the SNRs
of the eigenmodes, selects the rates for the eigenmodes, and
provides the selected rates to TX data processor 820.
[0098] The processing to transmit data on eigenmodes of the uplink
MIMO channel may be performed in a manner similar to that described
above for the downlink.
[0099] The rate selection techniques described herein may be
implemented by various means. For example, these techniques may be
implemented in hardware, software, or a combination thereof. For a
hardware implementation, the various units at the access point may
be implemented within one or more application specific integrated
circuits (ASICs), digital signal processors (DSPs), digital signal
processing devices (DSPDs), programmable logic devices (PLDs),
field programmable gate arrays (FPGAs), processors, controllers,
micro-controllers, microprocessors, electronic devices, other
electronic units designed to perform the functions described
herein, or a combination thereof. The various units at the user
terminal may also be implemented within one or more ASICs, DSPs,
processors, and so on.
[0100] For a software implementation, the techniques may be
implemented with modules (e.g., procedures, functions, and so on)
that perform the functions described herein. The software codes may
be stored in a memory unit (e.g., memory unit 842 or 882 in FIG. 8)
and executed by a processor (e.g., controller 840 or 880). The
memory unit may be implemented within the processor or external to
the processor, in which case it can be communicatively coupled to
the processor via various means as is known in the art.
[0101] The previous description of the disclosed embodiments is
provided to enable any person skilled in the art to make or use the
present invention. Various modifications to these embodiments will
be readily apparent to those skilled in the art, and the generic
principles defined herein may be applied to other embodiments
without departing from the spirit or scope of the invention. Thus,
the present invention is not intended to be limited to the
embodiments shown herein but is to be accorded the widest scope
consistent with the principles and novel features disclosed
herein.
* * * * *